A PWR fuel element is composed of a frame consisting of a top nozzle, a bottom nozzle, spacers, and guide tubes (or thimbles) and is filled with fuel rods. The size of a PWR lattice may vary from 14.times.14 to 19.times.19 rods. The primary function of the guide tubes is to provide space for control rods but they also function as supporting rods in the fuel assembly and as spacer holder rods. Usually, the guide tubes are made from a zirconium alloy of the same type as the cladding of the fuel rods.
During operation in a reactor, an extension of cladding and guide tubes takes place because of irradiation-induced growth. The growth is caused by radiation damage in the microstructure arranging itself in certain crystal planes in the hexagonal zirconium lattice and by the fact that the material therefore will grow perpendicular to these planes. The growth means that the fuel element all the time is extended under irradiation and that the element, after a certain irradiation, may become longer than the available space between the upper and lower core plates with an ensuing risk of the element bending or breaking. This means that the burnup of the element, which determines the residence time of the element in the core, must be limited.
The fuel element is designed such that the fuel rods are somewhat shorter than the space between the top and bottom nozzles, to provide a certain space for the fuel rods in which to grow. The guide tubes which serve as supporting rods are longer and must be attached to the top and bottom nozzles to hold the bundle together. In current designs, therefore, the growth of the guide tubes is determining for the burnup for which a fuel element can be designed.
According to EP-B-0 296 972, it is known to heat-treat cladding tubes for PWR fuel to reduce the corrosion and also the growth in the axial direction by heating the cladding tube to a temperature in the interval 950.degree. C. to 1250.degree. C., as a last step in the manufacture, and then rapidly cooling the tube, so-called beta-quenching. According to the patent the cladding shall be of a zirconium alloy, preferably Zircaloy-4 and of a certain composition within the scope of the specification for that alloy.
For fuel elements intended for boiling water reactors, BWR, it is known, according to U.S. Pat. No. 4,770,847, that fuel rods have a faster growth than water rods. To achieve a uniform growth of fuel rods and water rods, it is stated that a cladding tube with less tendency to grow can be manufactured by cold-working in a final step to achieve a suitable dimension, followed by heat treatment at about 538.degree.-705.degree. C. for 1 to 15 hours. A water rod with a somewhat higher tendency to grow is manufactured by heat-treating the tube, after cold working, at 440.degree.-510.degree. C. for 1 to 4 hours.